LIMA, Peru -- When a retired fisherman called to report that about 1,500 dolphins had washed up dead on Peru’s northern coast, veterinarian Carlos Yaipén’s first reaction was, “That’s impossible.”

But when Yaipén traveled up the coast last week, he counted 615 dead dolphins along a 135-kilometer stretch of coastline.

Now, the death toll could be as high as 2,800, based on volunteers’ counts. Peru's massive dolphin die-off is among the largest ever reported worldwide.

The strandings, which began in January, are a marine mystery that may never be unraveled. Experts say the causes could be acoustic impact from testing for oil or perhaps an unknown virus or other pathogen. Little marine research takes place in Peru, and even in the United States, of 55 marine mammal strandings since 1991, the National Oceanographic and Atmospheric Administration (NOAA) has classified 29 as “undetermined.”

All of the 20 or so animals Yaipén has examined showed middle-ear hemorrhage and fracture of the ear's periotic bone, lung lesions and bubbles in the blood. To him, that suggests that a major acoustic impact caused injury, but not immediate death. Most of the dolphins apparently were alive when they beached, or had died very recently.

“The animal would become disoriented, would have intense pain, and would have to make a great effort to breathe,” he said of the injuries.

Other experts say there is not enough evidence to draw a conclusion.

Stress or toxic contaminants can make marine mammals more vulnerable to pathogens such as viruses, according to Peter Ross, a research scientist at Canada’s Institute of Ocean Sciences in Sidney, British Columbia.

In a mass die-off, “there might be a smoking gun, but often we find that it’s two or three or four factors,” said Ross, who is one of the world’s leading experts on the effects of toxic contaminants in marine mammals.

Persistent organic pollutants, such as polychlorinated biphenyls or PCBs, the pesticide DDT, dioxins and flame-retardants accumulate in fish, and the concentrations are magnified as they move up the food web to top predators such as dolphins, seals and sea lions.

Laboratory studies of rodents and cells harvested from marine mammals show that PCBs and dioxins “are very immunotoxic,” Ross said. “The immune system is exquisitely sensitive to exposure to environmental contaminants.”

Animals with weaker immune systems could be more vulnerable to stress from noise or climate change, or to diseases such as leptospirosis, brucellosis or distemper, Ross said.

Scientists say that immune suppression from PCBs and DDT contributed to several marine mammal die-offs in the late 1980s and early 1990s, including dolphins along the Atlantic Coast and in the Mediterranean Sea, and harbor seals in northern Europe.

Yaipén, founder of the Lima-based Scientific Organization for the Conservation of Aquatic Animals (ORCA), knows of no studies of pollutants in Peru’s marine mammals. He has stored tissue samples from some of the beached dolphins but ORCA – a largely volunteer organization – has not yet been able to arrange for analysis.

Peru’s entire coast is a desert, its sandy beaches punctuated by peninsular cliffs and dotted with tiny fishing villages. On one trip up the coast, Yaipén said he initially counted a few dolphins every 150 meters, then every 10 or 20 meters.

The first account of 24 dead dolphins came on Jan. 21 in Piura -- on the north coast, just south of the border with Ecuador -- the same region where the 1,500 were reported by the staff of a marine coastal reserve on March 10. Another 416 were counted in Piura on March 21. More than 870 were spotted in February and March on beaches in the Lambayeque region, south of Piura.

Since it's ongoing, it may wind up being the largest dolphin die-off ever reported.

In 1987 and 1988, about 700 bottlenose dolphins died along the Atlantic coast from New Jersey to Florida. That may have depleted the coastal stock by more than 50 percent. Scientists concluded that the dolphins, which had bacterial and viral infections, were immune-suppressed.

Then, in the early 1990s, large numbers of striped dolphins – estimated at several thousand -- died in the Mediterranean Sea, starting in Spain. Infection by a morbillivirus was apparently the cause, but immune suppression was suspected, too, since the dead dolphins had higher concentrations of contaminants than ones that survived.

In Peru, two species have been stranded. About 90 percent are long-beaked common dolphins (Delphinus capensis), which swim close to the surface and have probably migrated south from Central America to feast on the abundance of fish in the nutrient-rich Humboldt Current that sweeps Peru’s coast.

The rest are Burmeister’s porpoises (Phocoena spinipinnis), a deepwater species that moves closer to the surface to calve. All the Burmeister’s porpoises Yaipén has recorded have been pregnant or lactating females or calves.

Yaipén worries that pathogens or contaminants in the dolphins could pose a health risk for residents of fishing villages along the coast, who have been cutting meat off the carcasses for food.

If they avoid the blubber, they will avoid most of the toxic chemicals, Ross said, but if the strandings are due to disease, they could be at risk of infection.

After sick and dead bottlenose dolphins washed up on the Louisiana coast recently, the U.S. Bureau of Ocean Energy Management (BOEM) ordered an oil and gas exploration company to suspend seismic testing – which uses air guns to map hydrocarbon deposits on the ocean floor – until May, when calving season ends.

Several oil leases under exploration are located off the coast or Peru where the dolphin strandings occurred, but it was not clear if seismic testing was under way. The offices of Savia Peru, which holds the leases, were closed Thursday, a national holiday in Peru.

A spokesman for Houston-based BPZ Energy said the company has been doing seismic testing since early February in an offshore lot several hundred kilometers north of where the dolphins were found.

Air guns can have “myriad impacts ... on marine mammals,” said Michael Jasny, senior policy analyst at the Natural Resources Defense Council, an environmental group that has urged BOEM to restrict seismic testing.

Although there has been more analysis of the impact of sonar than seismic testing, studies have linked loud ocean noises to ear and organ damage in marine mammals. Sounds can also change behaviors such as dive patterns – which can result in decompression sickness or “the bends” – or drive them closer to shore, where they could beach if they were disoriented.

“Lots of sound in the wrong place at the wrong time can lead to mass stranding,” Jasny said.

Environmental groups have gone to court several times to challenge the U.S. Navy’s use of sonar in military exercises, arguing that it can change marine mammal behavior and lead to strandings. There has been no information available on whether sonar has been used off Peru’s coast.

If noise is to blame in Peru, sonar could be a more likely culprit than seismic testing, according to Brandon Southall, former director of NOAA’s ocean acoustics program. He said the characteristics of the Peru strandings would be “atypical, but not impossible” for an acoustic-related stranding.

Even if sonar were a factor, the injuries may not be due directly to the impact of the sound. “Animals may react in a way that has a cascade of physiological effects,” Southall said.

This article originally ran at Environmental Health News, a news source published by Environmental Health Sciences, a nonprofit media company.